Prenatal diagnosis of neural tube defect before 12 weeks' gestation: direct and indirect ultrasonographic semeiology.

We describe the direct and indirect ultrasonographic features of a case of lumbar open spina bifida. The spinal defect was the prominent feature at 10 weeks + 5 days' gestation; however, cranial signs including narrowing of the frontal bones and flattening of the occiput were helpful at 12 weeks. This 'acorn' sign is likely to precede the 'lemon' sign, describing scalloping of the frontal bones at a later gestation. The diagnosis of spina bifida was confirmed by electrophoresis of the amniotic fluid, which showed an abnormal migration of acetylcholinesterase. Postmortem ultrasound examination of the same fetus proved useful in refining the diagnosis and also revealed the presence of the Arnold-Chiari malformation. Development of ultrasound screening in the first trimester of pregnancy should allow further evaluation of these findings. It seems reasonable to confirm such an early diagnosis by electrophoresis of the amniotic fluid as an alternative to ultrasonographic confirmation at 13-14 weeks.